The R-M420 family is defined most broadly by the SNPmutation M420, which was discovered after M17. The discovery of M420 resulted in a reorganization of the lineage in particular establishing a new paragroup (designated R-M420*) for the relatively rare lineages which are not in the R-SRY10831.2 (R1a1) branch leading to R-M17.

R-M420 and R-M17 are believed to have originated somewhere within Eurasia, most likely in the area from Central Europe to South Asia inclusive (see R-M17 for details on proposed origins).

The R-M420 family tree now has three major levels of branching, with the largest number of defined subclades within the dominant and best known branch, R1a1a (which will be found with various names; in particular, as "R1a1" in relatively recent but not the latest literature.)

R-M420, distinguished by several unique markers including the M420 mutation, is a subclade of Haplogroup R-M173 (previously called R1), which is defined by SNP mutation M173. Besides R-M420, R-M173 also has the subclades R-M343 (previously called R1b), defined by the M343 mutation, and the paragroup R-M173*. There is no simple consensus concerning the places in Eurasia where R-M173, R-M420 or R-M343 evolved.

R-M420, defined by the mutation M420, has two branches: R-SRY1532.2, defined by the mutation SRY1532.2, which makes up the vast majority; and R-M420*, the paragroup, defined as M420 positive but SRY1532.2 negative. (In the 2002 scheme, this SRY1532.2 negative minority was one part of the relatively rare group classified as the paragroup R1*.) Mutations understood to be equivalent to M420 include M449, M511, M513, L62, and L63.(Underhill 2009 and ISOGG 2012)

R-SRY1532.2 is defined by SRY1532.2, also referred to as SRY10831.2. SNP mutations understood to be always occurring with SRY1532.2 include SRY10831.2, M448, L122, M459, and M516 (Underhill 2009 and Krahn 2012). This family of lineages is dominated by the R-M17 branch, which is positive for M17 and M198. The paragroup R-SRY1532.2* is positive for the SRY1532.2 marker but lacks either the M17 or M198 markers.

The R-SRY1532.2* paragroup is apparently less rare than R1* but still relatively unusual, though it has been tested in more than one survey. Underhill 2009 for example report 1/51 in Norway, 3/305 in Sweden, 1/57 Greek Macedonians, 1/150 Iranians, 2/734 Ethnic Armenians, and 1/141 Kabardians.(Underhill 2009) While Sahoo 2006 reported R-SRY1532.2* for 1/15 Himachal Pradesh Rajput samples (Sahoo 2006).

R-M17 makes up the vast majority of all R-M420 over its entire geographic range. It is defined by SNP mutations M17 or M198, which have always appeared together in the same men so far. SNP mutations understood to be always occurring with M17 and M198 include M417, M512, M514, M515 (Underhill 2009). R-M17 has many subclades of its own defined by mutations. Two important subclades appear to broadly divide the European and Asian parts of this large clade:

Bryan Sykes in his book Blood of the Isles gives imaginative names to the founders or "clan patriarchs" of major British Y haplogroups, much as he did for mitochondrial haplogroups in his work The Seven Daughters of Eve. He named R1a1a in Europe the "clan" of a "patriarch" Sigurd, reflecting the theory that R1a1a in the British Isles has Norse origins.

The historic naming system commonly used for R1a was inconsistent in different published sources, because it changed often, this requires some explanation.

In 2002, the Y chromosome consortium (YCC) proposed a new naming system for haplogroups, which has now become standard.(YCC 2002) In this system, names with the format "R1" and "R1a" are "phylogenetic" names, aimed at marking positions in a family tree. Names of SNP mutations can also be used to name clades or haplogroups. For example, as M173 is currently the defining mutation of R1, R1 is also R-M173, a "mutational" clade name. When a new branching in a tree is discovered, some phylogenetic names will change, but by definition all mutational names will remain the same.

The widely occurring haplogroup defined by mutation M17 was known by various names, such as "Eu19", as used in (Semino 2000) in the older naming systems. The 2002 YCC proposal assigned the name R1a to the haplogroup defined by mutation SRY1532.2. This included Eu19 (i.e. R-M17) as a subclade, so Eu19 was named R1a1. Note, SRY1532.2 is also known as SRY10831.2[citation needed] The discovery of M420 in 2009 has caused a reassignment of these phylogenetic names.(Underhill 2009 and ISOGG 2012) R1a is now defined by the M420 mutation: in this updated tree, the subclade defined by SRY1532.2 has moved from R1a to R1a1, and Eu19 (R-M17) from R1a1 to R1a1a.

More recent updates recorded at the ISOGG reference webpage involve branches of R-M17, including one major branch, R-M417.

Contrasting family trees for R1a, showing the evolution of understanding of this clade